The present invention relates to elevators and, in particular, to a device for reducing transient vertical vibration acting on an elevator car.
A common problem associated with most elevators is that of low frequency vertical vibration of the elevator car. This phenomenon is principally due to the inherent elasticity of the main drive system used to propel and support the car within the hoistway; for example the compressibility of the working fluid used in hydraulic elevators and the elasticity of the rope used in traction elevators. Accordingly, any fluctuation in the force acting on the car will cause transient vertical vibration about a steady-state displacement of the car. The predominant frequency of these vibrations is that of the fundamental mode of vibration which is dependent on the travel height of the elevator and, for a traction elevator, the type of rope used. For a traction elevator having a travel path of 400 m and using steel ropes the fundamental frequency can be less than 1 Hz. Vibrations at such low frequencies are easily perceptible to passengers, undermining passenger confidence in the safety of the elevator and generally leading to deterioration in perceived ride quality.
There are two general sources of vibration, namely:
a) those due to fluctuations in the load of the car caused by embarkation and disembarkation of passengers while the car is held stationary by the drive at a landing; and
b) vibrations during travel caused by car overshoot during jerk phases of the drive, interference with other components within the elevator hoistway (wind forces due to passage of the car past shaft doors and neighboring cars within the hoistway, counterweight crossing, etc.) and movement of passengers within the traveling car.
The effects of the first of these sources of vibration are discussed in and addressed by European patent document EP 1 460 021 A1 where friction shoes mounted on the car are brought into contact with guide rails when the car is at rest at a landing. Hence, the overall damping ratio of the system is increased and the transient vibrations due to load fluctuations as passengers embark and disembark the car are attenuated more quickly. However, this solution is only applicable to a stationary elevator car and cannot solve the vibration experienced by a passenger in a traveling elevator car.
Furthermore, if the steady-state displacement of the car from the landing due to the change in the load is above a specific value, it may be necessary to perform a conventional re-leveling operation whereby the main drive is employed to make a small trip and thereby bring the car back to the level of the landing. The use of the main drive in this fashion, particularly since the car and landing doors are open, obviously presents an unwanted safety risk to passengers. The steady-state displacement must be determined before the re-leveling operation can commence, hence it necessarily has a slow reaction time. Furthermore, the re-leveling operation itself excites further low frequency vibrations.
One of the sources of vibration while the car is traveling is jerk phases in the travel curve of the drive. When a typical acceleration command generated by the elevator controller is fed directly into the motor of the main drive, there tends to be some overshoot in the car's response producing jerk and unwanted vibrations as shown by the first response curve R1 in FIG. 1. A conventional method of reducing the vibrations in the response is to compensate by rounding of the jerk as show by travel curve trajectory R2. However, this compensation of the response always increases travel time and therefore reduces the transport capacity of the elevator.
Furthermore, such compensation cannot solve the problem of vibrations induced by interference of the traveling car with other components within the elevator hoistway and movement of passengers within the car. In a traction elevator having a traction sheave driving a rope interconnecting the car and a counterweight, the sheave acts as a node in the fundamental mode of vibration particularly when the car is in the middle section of the hoistway and therefore has no influence whatsoever on the amplitude of the predominant fundamental vibrations experienced by the car. Until recently, this problem was not particularly disturbing to passengers traveling in the car since the ropes were relatively stiff being made from steel and therefore the amplitude of these vibrations was relatively small. However, with the development and subsequent deployment of synthetic ropes in traction elevators to replace traditional steel ropes, the elasticity of the ropes has approximately doubled and, for a travel path of 400 m, the fundamental frequency can be less than 0.6 Hz. This increase in elasticity combined with the decrease in the fundamental frequency makes the car much more susceptible to low frequency vertical vibrations. In particular, vibrations induced by interference of the traveling car with other components within the elevator hoistway and movement of passengers within the car are no longer a problem that can be disregarded since they will be increasingly perceptible to passengers in the future.